1. Field of the Invention
This invention relates to a vacuum deaerator for deaerating dissolved gas contained in water.
2. Description of the Prior Art
The prior art deaerators have been primarily developed for use with a condenser in a thermal power plant, while a boiled deaeration system including a heating step has been mainly adopted to this end. On the other hand, with a desalination plant of a multi-stage flash evaporation type, there have been generally adopted an external deaeration system, in which a deaerator is incorporated in a pretreatment system, including the step of decarbonation of the feed sea water introduced into a flash chamber, and a system, in which flash evaporation chambers are used as deaeration chamber. The former deaerator is of a packed column vacuum deaeration type which requires accessories such as deaerator, water delivery pump and the like, and thus is costly. On the other hand, in the adsence of stripping steam vapor, deaerated water of a low concentration may not be obtained, unless packing and like are increased in amount, so that the height of a column should be increased, thus resulting in a bulky plant. From viewpoints of operation, a high loading treatment incurs a flooding problem and the like, and thus the amount of brine to be treated should be limited to a range between 50 and 60 m.sup.3 /m.sup.2 h. The deaerator using the flash evaporation chambers utilizes flashing effect of feed sea water. However, it is a common practice to charge a packing, such as a tray, for obtaining deaerated water of a desired low concentration. However, this in turn poses a disadvantage in that the pressure loss due to charging of packings then lowers desired evaporation characteristics. In addition, a study has been under way to reduce the size of an evaporation chamber in a flash evaporator, by increasing the flow speed of brine. However, the reduction in size of the evaporation chamber leads to the increase in the height of a column for obtaining deaerated water of a low concentration, because liquid load on packing will be increased. In this case, difficulties are encountered when the shape of only the last stage evaporation chamber is changed, because of the arrangement of a heating pipe which has a considerable length and yet extends through 3 to 5 chambers. On the other hand, for providing temperature difference for the treated water, the number of the heat reject stage should be increased or a heater should be provided separately, resulting in the increase in cost of the apparatus. In addition, the flash temperature difference in the treated water is neutralized due to only one cycle of flashing, so that the temperature of the treated water after flashing will be equal to the temperature corresponding to a saturated vapor pressure within low pressure chambers. As a result, even if a vapor-to-liquid contacting area is widened and the dwelling time is extended, a less deaeration performance results, because of the least temperature difference between the treated water and the chambers. Thus, for obtaining deaerated water of a low concentration, the amount of packings should be increased or the load on treated water should be lessened.
It is accordingly an object of the present invention to provide a deaerator which is compact in size and deaerates brine to a lowered concentration by utilizing the flash deaerating effect of treated water according to a vacuum deaeration process.